RDS: Allow to create read only users

[flobaader]

Allow to create read-only users for dashboard or monitoring tools

Usecase: We use tools like Metabase to build dashboards for our postgres databases. Tools like this should get another user for the database that only has read-only access. This would be awesome to create with the already given syntax:

docker run \
    -e DB_INSTANCE_NAME=instance1 \
    -e DB_DATABASE_db1=database1 \
    -e DB_USERNAME_db1=user1 \
    -e DB_PASSWORD_db1=password1 \
    -e DB_READONLY_USERNAME_db1=password1 \
    -e DB_READONLY_PASSWORD_db1=password1 \
    pellepelster/solidblocks-rds-postgresql:v0.1.18